Generally, in an imaging device which forms images on a detector such as an image sensor using a white light source, an assistant light source has a very important role at low illuminance. Here, an image sensor refers to a semiconductor device configured to generate electric signals in response to light reflected by an object, that is, to convert an optical image into an electric signal, and a charge coupled device (CCD) and a complementary metal-oxide semiconductor (CMOS) image sensor are generally used as the image sensor.
In particular, the CMOS image sensor includes an amplifier in each cell of an equivalent circuit, has less noise in electric signals converted from light, and can realize actual colors through cells, which correspond to red/green/blue light and are arranged to intersect each other. In addition, since mass production of CMOS image sensors can be achieved through application of a CMOS logic LSI production processor, and the CMOS image sensors require low manufacturing cost and allow less power consumption due to a small size thereof as compared with CCD image sensors, the CMOS image sensors have mainly been used as components for DSLR cameras, which have been increasingly applied to camera phones and have been reduced in size in recent years.
However, since a CMOS image sensor applied to camera phones or advanced digital cameras has low detection sensibility at low illuminance, can easily become unstable and provides lots of noise in an image captured thereby, a camera flash also becomes important as an assistant light source at low illuminance.
FIG. 1 is a graph depicting spectral sensitivity of a CMOS image sensor for cameras. In this figure, blue, green, and red curves indicate sensitivity of the CMOS image sensor with respect to blue, green, and red light, respectively.
Referring to FIG. 1, spectral sensitivity of the CMOS image sensor has a wavelength range of a certain width with respect to each of the colors, in which the CMOS exhibits a lower spectral sensitivity with respect to blue light than other colors (green, red) and the highest spectral sensitivity with respect to red light.
Accordingly, in order to improve image quality of a digital camera, it is necessary for an assistant light source to have a spectrum wavelength distribution matching the spectral sensitivity of the CMOS image sensor with regard to the respective wavelengths. Such an assistant light source may be generally obtained by combination of a blue LED, a green phosphor and a red phosphor.
In mass production of light emitting devices, however, there can be deviation between the light emitting devices in terms of relative intensity between wavelength bands even when the light emitting devices emit light in a spectrum distribution matching the spectral sensitivity of the image sensor. Since it is difficult to adjust the relative intensity using electric current for operation, a light emitting device emitting light of unsatisfactory relative intensity between the wavelength bands will be determined to be a defective product. As a result, there is a problem of decrease of yield of light emitting devices for camera flashes.
Moreover, even when the spectrum distribution of the light emitting device matches the distribution of the spectral sensitivity of the CMOS image sensor, a spectrum distribution of light emitted from an assistant light source can often be different from that of ambient light. For example, when an image is captured using the assistant light source under illumination of relatively low color temperatures, the image includes an image of an object placed near a camera and illuminated by the assistant light source and an image of the background illuminated by ambient lighting. The object image and the background image have significantly different spectrum distributions due to a difference in spectrum distribution of the illumination light sources. Accordingly, unbalance occurs between the object image and background image, thereby providing a picture having an unnatural appearance.